Pupil: Aperture Of The Eye

One of the most important parts of the eye isn't a structure at all — it's an open space. It's the pupil of the eye.

Pupil Definition

The
pupil is the opening in the center of the iris (the structure that
gives our eyes their color). The function of the pupil is to allow light
to enter the eye so it can be focused on the retina to begin the process of sight.

Typically,
the pupils appear perfectly round, equal in size and black in color.
The black color is because light that passes through the pupil is
absorbed by the retina and is not reflected back (in normal lighting).

If
the pupil has a cloudy or pale color, typically this is because the
lens of the eye (which is located directly behind the pupil) has become
opaque due to the formation of a cataract. When the cloudy lens is replaced by a clear intraocular lens (IOL) during cataract surgery, the normal black appearance of the pupil is restored.

There's
another common situation when the pupil of the eye changes color — when
someone takes your photo using the camera's flash function. Depending
on your direction of gaze when the photo is taken, your pupils might
appear bright red. This is due to the intense light from the flash being
reflected by the red color of the retina. [Read more about red eyes in photos and how to avoid them.]

Pupil Function

Together,
the iris and pupil control how much light enters the eye. Using the
analogy of a camera, the pupil is the aperture of the eye and the iris
is the diaphragm that controls the size of the aperture.

The size of the pupil is controlled by muscles within the iris
— one muscle constricts the pupil opening (makes it smaller), and
another iris muscle dilates the pupil (makes it larger). This dynamic
process of muscle action within the iris controls how much light enters
the eye through the pupil.

In low-light conditions, the pupil
dilates so more light can reach the retina to improve night vision. In
bright conditions, the pupil constricts to limit how much light enters
the eye (too much light can cause glare and discomfort, and it may even
damage the lens and retina).

Pupil Size

The size of the
pupil varies from person to person. Some people have large pupils, and
some people have small pupils. Also, pupil size changes with age —
children and young adults tend to have large pupils, and seniors usually
have small pupils.

Generally, normal pupil size in adults ranges from 2 to 4 millimeters (mm) in diameter in bright light to 4 to 8 mm in the dark.

In
addition to being affected by light, both pupils normally constrict
when you focus on a near object. This is called the accommodative
pupillary response.

Pupil Testing

Typically,
pupil testing is performed in a dimly lit room. While you are looking
at a distant object, the examiner will briefly direct the beam of a
small flashlight at one of your eyes a few times. While doing this, the
response of the pupil of both eyes is observed.

The observer
typically will then alternately direct the light at each eye and again
observe the pupil responses of both eyes. This is called Marcus Gunn
pupil testing, which is sometimes called the "swinging flashlight test."

Pupils
normally react both directly and indirectly to light stimulation. The
reaction of the pupil of the eye receiving direct illumination is called
the direct response; the reaction of the other pupil is called the
consensual response.

The examiner may then turn up the room lights
a bit and have you focus on a hand-held object while moving that object
closer to your nose. This is a test of the accommodative response of
your pupils.

If your pupils appear normal and respond normally,
the clinician may record this popular acronym in your medical chart:
PERRLA, which is an abbreviation for "pupils are equal, round and
reactive to light and accommodation."

A pupil is abnormal if it fails to dilate in dim lighting or fails to constrict in response to light or accommodation.

Conditions That Affect The Pupil

A number of conditions can affect the size, shape and/or function of the pupil of the eye. These include:

Adie's tonic pupil.
This is a pupil that has nearly no reaction to light (direct or
consensual) and there is a delayed reaction to accommodation. Adie's
tonic pupil (also called Adie's pupil, tonic pupil, or Adie's syndrome)
usually affects only one eye, with the affected pupil being larger than
the pupil of the unaffected eye. The cause of Adie's pupil usually is
unknown; but it can be caused by trauma, surgery, lack of blood flow
(ischemia) or infection.

Argyll Robertson pupil.
This is a pupil that is not reactive to light (direct or consensual),
but reaction to accommodation is normal. Argyll Robertson pupil usually
affects both eyes, causing smaller-than-normal pupils that do not react
to light. The condition is rare and the cause usually is unknown, but it
has been associated with syphilis and with diabetic neuropathy.

Marcus Gunn pupil.
Also called relative afferent pupillary defect (RAPD) or afferent
pupillary defect, this is an abnormal result of the swinging-flashlight
test where the patient's pupils constrict less (therefore appearing to
dilate) when the light is swung from the unaffected eye to the affected
eye. The most common cause of Marcus Gunn pupil is damage in the
posterior region of the optic nerve or severe retinal disease.

Trauma.
Penetrating eye trauma that affects the iris is a common cause of
abnormally shaped pupils. Similar trauma can occur in complications of
cataract surgery, phakic IOL surgery or refractive lens exchange. Pupillary responses to light and accommodation often remain normal or nearly normal.

Sexual arousal.
Recent research has confirmed that sexual arousal elicits a pupil
dilation response, and that this response may be useful in sexuality
research to evaluate sexual orientation.